As far as I know, Keith, all the habitation ‘C’-Line sockets run off the leisure battery(ies) and the only sockets that run off the engine battery are those in the cab, which are not ‘C’-Line.
It’s easy to test which battery any socket runs off. Just disconnect the Earth from either the habitation or the engine battery, then try the socket… you’ll then know which battery is feeding it (or not!).
One idea that I’ve had done is to connect the cab sockets to the hab batteries rather than the engine battery – that way, whatever is plugged into the cab sockets won’t ever deplete the engine battery – you’ll always be able to start the engine.
Here are the various sockets, frames, surrounds in the ‘C’-Line range, where you can make up single, double, triple and more combinations… https://tinyurl.com/wg4ykqb